Feb. 13, 2008 -- Consistently overdoing it at the fast-food counter and leading an idle life may not bode well for the liver, not to mention the waist size.

So say Swedish researchers who studied 18 lean, healthy students -- mainly medical students -- who agreed to supersize themselves for science's sake.

The students were asked to gain 5% to 15% of their body weight in a month by eating at least two daily meals at fast-food restaurants and adopting a sedentary lifestyle. Their fast-food diet featured hamburgers and other foods high in saturated animal fat. The researchers reimbursed the cost of those meals.

Before-and-after measurements show a ballooning of the students' weight, waist, fat, and liver enzymes.

On average, the students gained 14 pounds, added 2.6 inches to their waistline, and padded their body fat percentage by 3.7% during the study.

Blood samples provided by the students throughout the study show a spike in levels of the liver enzyme alanine aminotransferase (ALT). ALT levels rose quickly -- typically within a week -- after the students started the fast-food diet.

High ALT levels can be a sign of liver damage. But other tests show that most of the students didn't develop fatty liver disease during the monthlong study.

The researchers -- who included Fredrik Nystrom, MD, PhD, of Sweden's University Hospital of Linkoping -- aren't focused on the occasional burger, but on a habitual pattern of being idle and overeating fattening foods from any source.

The study, published in the advance online edition of Gut, doesn't show which was more damaging -- bingeing on fatty food or being sedentary. And it doesn'tmean that all fast-food meals are bad choices. It's possible to make healthier choices at many fast-food chains.